Answer:
There are 28 marbles in the box. Some are red (12), others are yellow (6) and others are blue (10).
If in this box there 28 marbles and 6 of them are yellow, we substract to get the quantity of marbles that aren't yellow: 28-6 = 22
So 22 of the 28 marbles aren't yellow: 22/28, which we can simplify dividing the numerator and denominator by 2: 11/14